Tap water is not chemically pure H2O. Major additional components are calcium bicarbonate and sodium chloride, resulting from groundwater permeation through limestone (calcite) and salty rock layers (halite), respectively. Although these components do not pose a health risk (at least, in normal concentrations: for instance, in the Netherlands, the concentration of calcium bicarbonate may be in the order of 160 ppm and the concentration of sodium chloride may be in the order of 80 ppm), there are disadvantages associated with the presence of said components and therefore it is desirable to be able to control (particularly: reduce) their concentration levels.
Fouling is an issue. Calcium bicarbonate is the prime origin of scale deposits fouling for instance coffee makers, water boilers, steam irons and air humidifiers Minimizing the calcium bicarbonate concentration will delight customers because it avoids the periodic burden of having to clean these devices.
Taste is an issue. Even small quantities of minerals have a large impact on the taste perception. For instance the level of sodium chloride determines the saltiness of the tapwater. The taste of water is an important factor for a consumer to decide whether or not to drink it. Even if the quality of tap water is extraordinary good, people still tend to drink bottled water. Yet, bottled water is relatively expensive as compared to tap water, and its manufacture involves a much higher energy consumption as compared to tap water (in the order of a factor 1000, due to, among others, bottle manufacturing, transport and cooling). It is to be noted that consumers have individual taste preferences, and that there are large regional differences in tapwater content and taste. Therefore, with a view to influencing the taste of tap water so that consumers may switch from bottled water to tap water, it is important to be able to control the mineral level in tap water, and preferably to give the individual consumer a tool so that he can adapt the taste of tap water to his personal preference. It is further desirable that the control process is very effective. For instance, one specific brand of bottled water has a concentration of around 3 ppm of Na+ ions, while the detection threshold of Na+ ions is around 2 ppm: this means that a removal effectiveness of around 95% or higher is desirable.
Techniques for reducing the concentration of minerals in water have already been proposed, operating according to different principles. Generally, these conventional water purification techniques have disadvantages of high power consumption and waste generation.
A first conventional water purification technique to be mentioned here is distillation, which involves first to boil the water to produce vapour (excluding most minerals), and subsequently a cool surface is contacted where the vapour condenses as a liquid again. Although the effectiveness of this process is good (close to 100%), this process requires a lot of energy: it can be shown that the energy consumption is around 600 Wh/L, for compact units where energy recovery is not feasible.
A second conventional water purification technique to be mentioned here is reverse osmosis, which involves the use of high pressure (>10 bar) to force the water through a membrane filter with small pores to exclude the minerals. As compared to distillation, the energy consumption is significantly lower but still high (>4 Wh per liter output), and the effectiveness is typically around 90%. An important disadvantage is that a sizeable waste stream (typically 50% of input) is required in order to avoid clogging of the membrane. In addition, the membrane should be replaced regularly. For descaling applications, the 90% effectiveness of mineral removal is adequate, but for robust taste control a double pass would be required, at the cost of further reduced efficiency (12 Wh/L energy consumption, and 75% waste).
Conceptually both discussed purification techniques focus their energy on the H2O majority. In contrast, with electric fields it is possible to target the mineral minority efficiently, exploiting the fact that in water these species are ionized. Examples of such techniques are continuous electro de-ionization and capacitive de-ionization.
In continuous electro de-ionization, a stack of parallel streams—separated by ion-selective membranes, and partly filled with ion exchange resins—is electrically converted into purified streams and concentrated waste streams. This purification technology (usually as final clean-up step following reverse osmosis) can create ultrapure water to the highest standards and with excellent efficiency (0.4 Wh/L energy consumption and only 5% waste), but due to the high cost of the membranes and resins it is out of reach for consumer use.
In capacitive de-ionization, an incoming stream is purified by electrically capturing its ions onto two parallel plate electrodes (constructed from porous carbon). This technology is operated in a batch process, alternating between de-ionization and regeneration phases. After the electrodes are saturated with adsorbed ions, in the regeneration phase the electric field is removed and the ions gradually diffuse from the electrodes into the waste stream. Disadvantage of this technology is that the regeneration phase is slow and incomplete. Consequently the waste stream may be high (>20%), and the lifetime of the electrodes will be limited (due to biofouling). An example of this technique is addressed in US-2008/0198531.
WO-2008/082696 discloses an electrical water purification technique indicated as “Microscale Capacitive Deionization”. In this technique, a stream of water is subjected to a transverse electric field applied by electrodes arranged along the water channel. Ions are attracted to the charged electrodes, so that the ion concentration in the center of the stream is reduced and the ion concentration in the outer regions of the stream, close to the electrodes, is increased. After some length of channel, the central stream part is separated from the outer stream regions, the central stream providing the output with reduced ion concentration and outer stream regions providing a waste stream with increased concentration. This technique seems very unattractive: according to the information in the publication, very high voltages are needed (in the order of 2.5 to 10 kV), the power consumption is very high (750 W for 50 mL/hr, corresponding to 15 kWh/L), and the ion purification performance is very poor (around 3%).